This application is based on Japanese Patent Application No. 11-93463 (1999), the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a semi-transmissive liquid crystal display device having a semi-transmissive film. More particularly, the invention relates to a liquid crystal display device operable both in a reflective mode and in a transmissive mode.
2. Description of the Related Art
In recent years, liquid crystal display devices have widely been employed as display devices for small-and medium-size notebook personal computers and other portable information terminals and as large-scale monitors. In general, the liquid crystal display devices for the portable information terminals are semi-transmissive STN liquid crystal display devices for indoor and outdoor use.
Such a semi-transmissive liquid crystal display device is operative in a reflective mode in external illumination such as sunlight or fluorescent light, and in a transmissive mode which utilizes a backlight as internal illumination. For the operation in the reflective and transmissive modes, use of a semi-transmissive film has been proposed (see Japanese Unexamined Patent Publication No. 8-292413 (1996), for example).
The semi-transmissive film is conventionally comprised of a thin metal film such as of aluminum or chromium. In the reflective mode, the semi-transmissive film functions as a reflective film. In the transmissive mode, the semi-transmissive film functions as a transmissive film.
Use of a semi-transmissive film in a liquid crystal display device of active matrix type has also been proposed (see Japanese Unexamined Patent Publication No. 7-318929 (1995), for example). The semi-transmissive film for use in the active matrix liquid crystal display device is comprised of a thin metal film such as of aluminum or an aluminum-based alloy, or a laminate film comprising a transparent conductive film such as an ITO film and a metal film.
In the conventional semi-transmissive liquid crystal display devices, the semi-transmissive films do not sufficiently satisfy the requirements for the light transmitting property and the light reflecting property. In other words, the conventional semi-transmissive liquid crystal display devices cannot offer satisfactorily improved performance in the reflective and transmissive modes.
To overcome the aforesaid drawback, it is a principal object of the present invention to provide a high performance semi-transmissive liquid crystal display device which offers satisfactorily improved performance both in the reflective mode and in the transmissive mode.
It is another object of the invention to provide a semi-transmissive film suitable for use in the semi-transmissive liquid crystal display device.
It is further another object of the invention to provide a semi-transmissive liquid crystal display device which features a greater viewing angle and a greater viewable area for image display.
The liquid crystal display device according to the present invention is operable both in the reflective mode and in the transmissive mode. For this purpose, the liquid crystal display device has a semi-transmissive film which functions as a reflective film in the reflective mode, and as a transmissive film in the transmissive mode.
The semi-transmissive film is totally composed of a dielectric material. More specifically, the semi-transmissive film has a multi-layered laminate structure which comprises high refractivity dielectric layers and low refractivity dielectric layers. The high refractivity layers and the low refractivity layers are simply different in refractivity. When the liquid crystal display device with the semi-transmissive film having such a construction is operative in the reflective mode, light rays incident on the semi-transmissive film are partly reflected by the high refractivity layers, and light rays transmitted through the high refractivity layers are reflected by the low refractivity layers. The reflected light rays interfere with each other, so that so-called reflection enhancement occurs to remarkably enhance the reflecting property of the semi-transmissive film. Therefore, the liquid crystal display device ensures clear image display in a liquid crystal layer with little reduction in brightness thereof.
When the liquid crystal display device is operative in the transmissive mode, light rays from a backlight pass through the semi-transmissive film to enter a liquid crystal panel, and exit from a display surface side of the liquid crystal display device. Therefore, the liquid crystal display device also ensures clear image display in the transmissive mode.